1. Field of the Invention
The present invention relates to a printer, and more particularly, to a color printer for repetitively printing a document.
2. Description of the Prior Art
Repetitive printing methods are commonly used by color printers in printing color documents. When printing a color document, a color printer will usually print one color in each printing pass and three passes are usually required for printing three different colors. Such repetitive printing methods require accurate printing position control so that different colors can be accurately printed to form a complete color document.
Please refer to FIG. 1. FIG. 1 is a structural diagram of part of a prior art printer 10. The printer 10 comprises two cylindrical roller wheels 12, a stepping motor 14, a detector 16, a printing head 18, a reduction gear 20, and a paper supplier 22. The two roller wheels 12 are rotatably positioned together and in contact with each other for clamping and carrying a document 24 supplied by the document supplier 22. The reduction gear 20 is installed between the stepping motor 14 and the two roller wheels 12 so that the stepping motor 44 can drive the two roller wheels 12 forward or backward through the reduction gear 20. The detector 16 is installed at one side of the two roller wheels 12 for detecting a front edge 26 of the document 24. The printing head 18 is installed at another side of the roller wheels 12 for printing the document 24 after the detector 16 detects the front edge 26 of the document 24.
Please refer to FIG. 2. FIG. 2 is a function block diagram of the printer 10 shown in FIG. 1. The printer 10 comprises a stepping motor 14, a detector 16, a printing head 18, and a control circuit 30 which has a memory 32. The control circuit 30 is connected to the stepping motor 14, detector 16 and printing head 18 for controlling operations of the printer 10. The memory 32 is used for recording the number of forward or backward steps of the stepping motor 14 for controlling the printing position of the document 24. When the detector 16 detects the front edge 26 of the document 24, it sends a signal to the control circuit 30 so that the control circuit 30 can set an initial printing position in the memory 32. The control circuit 30 then controls the printing head 18 and stepping motor 14 to initiate a color printing pass. When the first color printing pass is finished, the control circuit 30 will activate the stepping motor 14 to carry the document 24 backward the number of forward steps recorded in the memory 32, and then repeats the color printing pass until all the colors are printed.
The printing method of the prior art printer 10 in printing a color document can be summarized below:
a. activating the stepping motor 14 to carry the document 24 forward until the detector 16 detects the front edge 26 of the document 24; PA1 b. setting an initial printing position in the memory 32; PA1 c. activating the stepping motor 14 to carry the document 24 forward and using the printing head 18 to print the document 24; recording the number of forward steps in the memory 32 until the current printing pass is finished; PA1 d. activating the stepping motor 14 to carry the document 24 backward the number of forward steps recorded in the memory 32; PA1 e. repeating the above steps until all colors are printed. PA1 two cylindrical roller wheels rotatably positioned together and in contact with each other for clamping and carrying the document forward and backward; PA1 a stepping motor for driving the two roller wheels forward and backward; PA1 a detector installed at one side of the two roller wheels for detecting a front edge of the document; PA1 a printing head installed at another side of the two roller wheels for printing the document; and PA1 a control circuit connected to the stepping motor, detector and printing head for controlling operations of the printer, the control circuit comprising a memory for recording the number of forward or backward steps of the stepping motor as printing position of the document; PA1 (1) using the stepping motor to carry the document forward and using the detector to detect the front edge of the document; PA1 (2) calculating the number of forward steps of the stepping motor after the front edge of the document is detected and using the printing head to print the document according to the calculated forward steps until one pass of the printing is finished; PA1 (3) carrying the document backward to move the front edge of the document away from the detector; and PA1 (4) repeating from the step (1) to the step (3) until all passes of the printing are finished.
One problem experienced in the prior art printer 10 is that when the document 24 is carried forward or backward by the two roller wheels 12, the frictional force between the document 24 and the printer 10 may drag the document 24 and thus cause small displacements between the document 24 and the two roller wheels 12. The small displacements will be accumulated in each printing pass which will make the printing position recorded in the memory 32 inaccurate. Such inaccuracy will cause the colors of the following printing passes printed in wrong places.